In this work, extensive electrochemical impedance measurements have been conducted on a 45 cm(2) BASF Celtec P2100 high temperature PEM MEA. The fuel cell performance has been examined subject to some of the poisoning effects experienced when running on a reformate gas. The impedance is measured at different temperatures, currents, and different content of CO, CO2 and H-2 in the anode gas. The impedance spectrum at each operating point is fitted to an equivalent circuit and an analysis to identify the different mechanisms governing the impedance is performed. The trends observed, when varying the operating conditions under pure H-2, generally show good agreement with results from the literature. When adding CO and CO2 to the anode gas the entire frequency spectrum is affected, and especially the measurements conducted at low temperatures and high CO concentrations reveal undesirable transient effects. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.